A calibrating operation of a camera is to obtain intrinsic camera parameters that should be done in advance for every operation of acquiring three-dimensional (3D) information by using the camera, and can be widely used for computer vision, robotics, film industry, human-computer interaction, etc.
Calibration methods of a camera may be roughly divided into three as follows:
The first method is to use a calibration pattern specially made for the camera calibration. This enables to precisely detect a relative position of a specific marker on the calibration pattern in advance, thereby getting an accurate result. However, this method has a disadvantage in that a heavy and large calibration pattern has to be carried all the time.
The second method is to take a lot of continuous images and utilize them for the calibration of a camera. This is a method based on projective geometry reconstruction, which is called auto-calibration, and uses the assumption that some of the intrinsic parameters of the camera are known. Although this method can be carried out without any geometric information about a specific calibration pattern or a scene within an image, it has a disadvantage in that a lot of images are required for accuracy.
The third method is to calibrate a camera by using geometric information existing in the scene within an image taken by the camera. The geometric information existing in the scene refers to a set of two parallel lines perpendicular to each other, a sphere, a circle, etc. With this information, no specific calibration pattern is required, and the camera can be precisely calibrated without using a lot of images.
Among the conventional methods for calibrating a camera, the method for calibrating a camera by using geometric information existing in the scene within an image taken by the camera has the above-described advantages, but has the problem of being unable to calibrate the camera if there exists no geometric information, such as a set of two parallel lines perpendicular to each other, a sphere, a circle, etc., in the scene within the image taken by the camera.